1. Field of the Invention
The present invention relates to a light-emitting-element array, more particularly to its wire-bonding interconnection pads and their interconnections to the light-emitting elements.
2. Description of the Related Art
The type of light-emitting-element array with which the present invention is concerned has a plurality of light-emitting elements spaced at equal intervals. A light-emitting-element array using light-emitting diodes (LEDs) as its light-emitting elements is referred to as an LED array. Conventional LED arrays have, for example, the structure shown in FIGS. 18 and 19, which are taken from page 60 of ‘LED Purinta no Sekkei’ (Design of LED Printers), published by Triceps.
FIG. 18 is a sectional view of the relevant parts of an LED array 100, and FIG. 19 is a plan view. The LED array 100 comprises an n-type gallium arsenide phosphide (GaAsP) layer 101 formed on an n-type gallium arsenide (GaAs) substrate 105. A p-type impurity such as zinc (Zn) is selectively diffused into the GaAsP layer 101 to form a row of p-type regions 106 that function as light-emitting elements. The GaAsP layer 101 is covered by a dielectric film 102 with openings through which the tips of aluminum electrodes 103 make individual contact with the p-type regions 106.
In order to form an electrical connection with a driver circuit (not shown), each of the other ends of the aluminum electrodes 103 is electrically coupled to an electrode pad 107 formed to present a flat surface with an adequate area for wire bonding. A gold-germanium-nickel (AuGeNi) electrode 104 is formed under the n-type GaAs substrate 105 as a common electrode, electrically coupled to the p-type regions 106 through the n-type GaAs substrate 105 and GaAsP layer 101.
In many conventional LED arrays, each light-emitting element has a separate electrode pad 107, as shown. A dense array of light-emitting elements therefore has a correspondingly dense set of electrode pads, but the density of the electrode pads makes wire bonding difficult, leading to a rise in manufacturing cost.
The necessary number of electrode pads can be reduced by a matrix driving scheme, but known matrix driving schemes also lead to low manufacturing yields and correspondingly high manufacturing costs. The low yields are due to interconnection faults such as short circuits and open circuits that may occur where interconnecting lines cross one another, or cross isolation trenches.